Saute Shallots until translucent and tender. Add peas not set aside, 4 cups broth and 2tbsp. tarragon. Bring to a boil. Reduce heat to medium and boil until flavors blend and peas are tender 7-10 min. Cool slightly. Puree soup in batches in a blender (or use immersion blender) until completely smooth. Return soup to pan and return to simmer. If desired, use remaining broth to thin soup. Add remaining tarragon.

Heat cream over low heat until about 100 degrees. Mix in buttermilk, stirring well. Put mixture in covered jar and leave at room temperature for about 8 hours. Refrigerate for at least 24 hours before using. Will keep in refrigerator up to 3 weeks.